std::is_constant_evaluated
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| Defined in header <type_traits>
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| constexpr bool is_constant_evaluated() noexcept; |
(since C++20) | |
Detects whether the function call occurs within a constant-evaluated context. Returns true if the evaluation of the call occurs within the evaluation of an expression or conversion that is manifestly constant-evaluated; otherwise returns false.
To determine whether initializers of following variables are manifestly constant-evaluated, compilers may first perform a trial constant evaluation:
- variables with reference type or const-qualified integral or enumeration type;
- static and thread local variables.
It is not recommended to depend on the result in this case.
int y = 0; const int a = std::is_constant_evaluated() ? y : 1; / Trial constant evaluation fails. The constant evaluation is discarded. / Variable a is dynamically initialized with 1 const int b = std::is_constant_evaluated() ? 2 : y; / Constant evaluation with std::is_constant_evaluated() == true succeeds. / Variable b is statically initialized with 2
Contents |
[edit] Parameters
(none)
[edit] Return value
true if the evaluation of the call occurs within the evaluation of an expression or conversion that is manifestly constant-evaluated; otherwise false.
[edit] Possible implementation
/ This implementation requires C++23 if consteval. constexpr bool is_constant_evaluated() noexcept { if consteval { return true; } else { return false; } } |
[edit] Notes
When directly used as the condition of static_assert declaration or constexpr if statement, std::is_constant_evaluated() always returns true.
Because if consteval is absent in C++20, std::is_constant_evaluated is typically implemented using a compiler extension.
| Feature-test macro | Value | Std | Feature |
|---|---|---|---|
__cpp_lib_is_constant_evaluated |
201811L |
(C++20) | std::is_constant_evaluated
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[edit] Example
#include <cmath> #include <iostream> #include <type_traits> constexpr double power(double b, int x) { if (std::is_constant_evaluated() && !(b == 0.0 && x < 0) { / A constant-evaluation context: Use a constexpr-friendly algorithm. if (x == 0) return 1.0; double r {1.0}; double p {x > 0 ? b : 1.0 / b}; for (auto u = unsigned(x > 0 ? x : -x); u != 0; u /= 2) { if (u & 1) r *= p; p *= p; } return r; } else { / Let the code generator figure it out. return std::pow(b, double(x)); } } int main() { / A constant-expression context constexpr double kilo = power(10.0, 3); int n = 3; / Not a constant expression, because n cannot be converted to an rvalue / in a constant-expression context / Equivalent to std::pow(10.0, double(n)) double mucho = power(10.0, n); std::cout << kilo << " " << mucho << "\n"; / (3) }
Output:
1000 1000
[edit] See also
constexpr specifier(C++11)
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specifies that the value of a variable or function can be computed at compile time[edit] |
consteval specifier(C++20)
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specifies that a function is an immediate function, that is, every call to the function must be in a constant evaluation[edit] |
constinit specifier(C++20)
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asserts that a variable has static initialization, i.e. zero initialization and constant initialization[edit] |
